CONTROL OF SINGLE STAND/REVERSING MILLS

Abstract: A single stand strip rolling mill is controlled by a signal representing the speed of the strip exiting the rolling mill. The rolling mill is controlled to keep the exit strip speed substantially constant. In this way the entry strip speed is kept substantially constant and gauge variations in the rolled strip are reduced.

Description: CONTROL OF SINGLE STAND/REVERSING MILLS This invention relates to the operation of a single stand rolling mill to roll metal strip and to the operation of a single stand rolling mill as a reversing mill to roll metal strip. A single stand rolling mill for rolling metal strip is normally set to run at a rolling speed determined by the mill operator. After the strip has been threaded into the mill, the mill is accelerated up to a speed level which is fixed by the operator. The mill speed remains at this level while most of the strip is rolled and, when the tail end of the strip approaches the mill, the speed is reduced in order for the tail end to be rolled and handled without damage. To adjust the speed of the mill, the operator alters a mill speed reference signal applied to the closed loop speed control system and the control system follows this reference. It is usual for the rolling mill to have its rolls rotated at a constant speed and the uncoiler from which the strip is uncoiled and the coiler onto which the strip is coiled then take up appropriate speeds which are determined by the entry/exit speeds of the mill and the tension levels between the uncoiler and the mill and between the mill and the coiler, respectively. If one of these tension levels changes, the speed of the respective uncoiler or coiler is changed to re-establish the original tension level. Mill speed is usually the term applied to the circumferential speed of the mill rolls and is usually taken to be an indicator of the speed of the outgoing rolled strip; however, this is not strictly true because there is "forward slip" between the rotating rolls and the strip. This is created by the mechanics of the roll bite and it changes under the influences of; lubrication between the rolls and the strip; of rolling load and of tension deviations. Roll load variations occur as a result of changes in strip entry thickness requiring load changes needed to remove these thickness variations. Speed variations in outgoing strip from the rolling mill necessitate corresponding variations in the speed of rotation of the coiler. The inertia of a coiler varies in dependence of the thickness of the coil of strip which is wound on the coiler, the inertia being a maximum for a full coil. This variation in inertia puts a strain on the drive motor for the coiler and a high inertia increases the response time of the control circuit. It is an object of the present invention to operate a single stand rolling mill such that unwanted effects of slip variation on the quality of the rolled strip are reduced. According to the present invention, in a method of controlling the operation of a single stand strip rolling mill the strip leaving the mill is coiled on a coiler and the speed of rotation of the mill rolls are adjusted to provide a substantially constant speed for the strip leaving the mill. An advantage of the invention is to keep a substantially constant coiling speed thereby reducing the demands of the exit tension controller, thereby holding more constant the level of outgoing tension, thereby producing strip of a more constant thickness and coiled tension. Alternatively, the mill could be controlled so as to keep the ratio of entry material speed V and outgoing material speed v essentially constant so that reduced variations in outgoing speed cause a reduction in entry speed variations. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a schematic circuit diagram of the usual drive arrangement for a single stand rolling mill; and Figures 2 and 3 are schematic circuit diagrams showing the drive arrangement for a single stand rolling mill in accordance with alternative embodiments of the present invention. A single stand rolling mill 1 has its work rolls driven by separate drive shafts from a drive motor 3 via a gear box 5. A coil of strip 7 on a driven pay-off reel 9 is threaded through the mill and on to a coiler 11. Tachometers 13 and 15 driven by the strip measure the ingoing and outgoing speeds of the strip, respectively. The main drive motor 3 is connected to a tachometer 17 and a signal from this tachometer is compared in a comparator 19 with a signal from an adjustable speed reference source 21. The difference signal is used to control a thyristor power unit 23 in the sense to adjust the motor speed to the value set by the reference source. Although the speed of rotation of the mill rolls is kept substantially constant by the power unit 23, the linear speed of the outgoing strip from the mill changes due to the variation of forward slip. Referring to Figure 2, the tachometer 15 , which provides a signal proportional to the speed of the strip immediately prior to it being coiled, is connected to the comparator 19 instead of the main drive tachometer. In the comparator the signal is compared with the speed reference signal from the source 21. Any difference between these signals is amplified and fed to the thyristor power unit 23 where it is used to control the operation of the motor 3. Thus, the exit speed of the strip material is kept substantially constant and the level of outgoing tension is held more constant. The relationship between the ingoing and outgoing speed of the strip is substantially constant so entry speed changes are reduced. In an alternative embodiment, shown in Figure 3, the main drive motor tachometer 17 is retained in the motor control loop and a trim signal added to the speed reference signal from the source 21. The comparator 19 receives signals from the tachometer 17 and a speed reference 21 and also a trim signal obtained indirectly from the exit tachometer 15. This signal from the tachometer 15 and the speed reference signal are applied to a control amplifier whose output representing speed deviation serves as the trim signal. This trim signal is applied to the main drive motor control loop in order to hold constant the exit speed for small perturbations in exit speed. Large perturbations in exit speed will cause main drive speed trim to hold steady (as the control amplifier 24 saturates) and then the coiler is allowed to accelerate taking the main drive speed trim into its active region. In similar fashion, a high pass filter 25 could be fitted into the main drive speed trim line thus allowing the main drive motor to compensate for short term perturbations in exit speed and the exit coiler to compensate for long term perturbations in exit speed. These alternatives are shown by the routing of exit speed signal in the dotted or dashed lines. As a result of the invention, the mill exit speed is kept substantially constant during the rolling of all but the front and tail ends of the strip during which the strip is deliberately accelerated and decelerated respectively. The entry speed of the strip is also kept substantially constant. This results in reduced entry tension changes and gauge errors of strip rolled in the mill are reduced.

Claims: Claims : 1. A method of controlling the operation of a single stand strip rolling mill where the strip leaving the mill is coiled on a coiler and where the speed of rotation of the mill rolls are adjusted to provide a substantially constant speed for the strip leaving the mill. 2. A method as claimed in claim 1 where a signal representing the speed of the strip at a position between the mill and the coiler is compared with a signal representing the desired speed of the strip leaving the mill to produce an error signal which is used to control the speed of rotation of the mill rolls in the sense to reduce said error signal substantially to zero. 3. A method as claimed in claim 1 wherein a signal representing the speed of rotation of the mill rolls is compared with a signal representing the desired speed of the strip leaving he mill and a signal representing the deviation of speed of the strip at a position between the mill and the coiler to produce an error signal which is used to control the speed of rotation of the mill rolls in the sense to reduce said error signal substantially to zero. 4. A method as claimed in claim 3 where the signal representing the speed deviation of the strip is limited to a predetermined level. 5. A single stand strip rolling mill, means for driving the mill rolls, a driven coiler downstream of the stand, means for determining the speed of the strip material between the mill stand and the coiler, and control means including comparator means for comparing the speed of the strip material with a desired speed of the strip material to produce an error signal which serves to control the driving means for the mill rolls. 6. A single stand strip mill as claimed in claim 5 including means for producing a signal representing the speed of rotation of the mill rolls, said means being connected to supply said signal to the comparator means. 7. A single stand strip mill as claimed in claim 6 in which the signal representing the deviation of speed of strip material between the mill stand and the coiler is limited to a predetermined maximum value.

Inventors: BURNAND PETER WILLIAM, BURNAND, PETER, WILLIAM
Applicants: BURNAND PETER WILLIAM, DAVY MCKEE SHEFFIELD, BURNAND, PETER, WILLIAM, DAVY MCKEE (SHEFFIELD) LIMITED
Application year: 1994
Main classification: B21B37
IPC codes: B21B1, B21B37